The present invention relates to a field effect transistor usable as a high-frequency high-output-power transistor and a method for fabricating the same.
A group III-V nitride compound semiconductor typified by GaN (represented by a general formula, InxAlyGa1-x-yN (wherein 0≦x≦1 and 0≦y≦1) hereinafter referred to as InAlGaN) is regarded as a promising material for a high-frequency high-output-power electronic device because it has a wide band gap (of, for example, 3.4 eV at room temperature in GaN), a very large breakdown electric field and very high saturated electronic velocity. In particular, in a hetero junction structure obtained by stacking an AlGaN film on a GaN film (hereinafter referred to as an AlGaN/GaN hetero structure), electrons are stored in a high concentration in the vicinity of the hetero junction interface within the GaN film owing to a strong polarization electric field formed on the (0001) plane, so as to generate what is called a two-dimensional electron gas. Since the two-dimensional electron gas is three-dimensionally separated from a donor impurity included in the AlGaN film and hence has high electron mobility. Furthermore, a GaN-based material is high at what is called saturated drift velocity, and has electronic velocity twice or more as high as that of a GaAs-based material currently widely used as a material for a high-frequency transistor in a high electric field region of, for example, approximately 1×105 V/cm. Also, the GaN-based material has a large band gap and a large breakdown electric field. Therefore, it is expected to be applied to a high-frequency high-output-power device.
In a field effect transistor having the AlGaN/GaN hetero structure (hereinafter referred to as the HFET (heterojunction field effect transistor)), however, high-frequency characteristics expected based on the material properties have not been attained yet. One of the reasons is large parasitic resistance derived from ohmic resistance, which will now be specifically described. The characteristic parameters of the HFET are cut-off frequency fT and mutual conductance gm, and there is a relationship represented by the following formula 1 between these parameters:fT=gm/(2πLg)  Formula 1:wherein Lg is a gate length. As is obvious from Formula 1, it is significant to increase the mutual conductance gm for improving the cut-off frequency fT. In consideration of parasitic resistance, the mutual conductance gm is represented by the following formula 2:gm=gmint/(1+Rs·gmint)  Formula 2:wherein gmint is intrinsic mutual conductance determined depending upon the material and the structure and Rs is parasitic resistance from an ohmic electrode to a channel and is designated as source resistance. As is understood from Formula 2, the mutual conductance gm increases as the source resistance Rs reduces, and as a result, the cut-off frequency fT also increases. Accordingly, in order to improve the high-frequency characteristics of the HFET, it is necessary to reduce the parasitic resistance.
In an AlGaN/GaN-based HFET, an ohmic electrode is formed on AlGaN with a larger band gap, and hence it is difficult to reduce contact resistance. Therefore, a cap layer made of a material with a smaller band gap is conventionally sandwiched between the electrode and the AlGaN so as to reduce the contact resistance. Such a cap layer is conventionally made of GaN (see Non-patent Document 1). In this conventional method, however, a potential barrier is caused between the GaN and the AlGaN because there is a difference in the polarization between the GaN and the AlGaN. As a result, parasitic resistance is increased on the GaN/AlGaN interface, so as to disadvantageously degrade the high-frequency characteristics.
In order to overcome this disadvantage, Patent Document 1 proposes a cap layer made of InAlGaN having a composition matching in lattice with GaN and having larger polarization than AlGaN.
FIG. 9A is a cross-sectional view of a conventional field effect transistor disclosed in Patent Document 1. As shown in FIG. 9A, an undoped GaN buffer layer 102 is formed on a sapphire substrate 101 by epitaxial growth. A two-dimensional electron gas 107 is generated in an upper portion of the undoped GaN buffer layer 102, so that the upper portion of the undoped GaN buffer layer 102 can function as a channel layer of the field effect transistor. An n-type AlGaN electron supply layer 103 and an n-type InAlGaN cap layer 104 are successively formed on the undoped GaN buffer layer 102 by the epitaxial growth. A recess reaching the n-type AlGaN electron supply layer 103 is formed in a given portion of the n-type InAlGaN cap layer 104, and a Schottky electrode 106 made of Pd—Si (namely, an alloy of Pd and Si) working as a gate electrode is formed in the recess. Also, ohmic electrodes 105 made of Ti/Al (namely, having a multilayered structure including a Ti layer and an Al layer) working as a source electrode and a drain electrode are formed on the n-type InAlGaN cap layer 104 on the respective sides of the Schottky electrode 106.
FIG. 9B shows change of Al and In composition along line Y-Y′ of FIG. 9A, and FIG. 9C shows change of electron potential energy along line Y-Y′ of FIG. 9A.
Non-patent Document 1: T. Egawa, et al., Recessed gate AlGaN/GaN modulation-doped field-effect transistors on sapphire, Applied Physics Letters Vol. 76, pp. 121-123, U.S.A., American Institute of physics, Jan. 3, 2000
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-289837